U.S. patent number 5,530,613 [Application Number 08/251,880] was granted by the patent office on 1996-06-25 for current limiting circuit controller.
This patent grant is currently assigned to Eaton Corporation. Invention is credited to James A. Bauer, Denis A. Mueller, James D. B. Smith, Henry A. Wehrli, III.
United States Patent |
5,530,613 |
Bauer , et al. |
June 25, 1996 |
Current limiting circuit controller
Abstract
A circuit controller for selectively connecting a power source
to a load includes a non-overcurrent protective electrical
switching mechanism and a conductive polymer current limiter
connected in series with the load for limiting the current which
flows through the switching mechanism whenever the power source is
selectively connected to the load. The conductive polymer current
limiter may include a plurality of parallel conductive polymer
positive temperature coefficient (PTC) resistors. The circuit
controller may include an electrical switching mechanism which,
under short circuit conditions, remains selectively connected to
the load. The short circuit current which is limited by the
conductive polymer current limiter continues to flow through the
electrical switching mechanism. The switching mechanism may include
a contactor having separable contacts, and may further include a
circuit interrupter, in order to provide a separate circuit
disconnection mechanism. The electrical switching mechanism may
have a coil and the circuit controller may include a voltage
sensor. The conductive polymer current limiter may have an output
which has a voltage substantially less than the voltage of the
power source whenever the short circuit current flows from the
power source and through the current limiter. The voltage sensor
may be connected to the output of the conductive polymer current
limiter and may deenergize the coil in order to open the separable
contacts of the contactor and disconnect the power source from the
load whenever the voltage of the output drops below a predetermined
threshold. The contactor may be a motor-starting contactor.
Inventors: |
Bauer; James A. (Asheville,
NC), Mueller; Denis A. (Asheville, NC), Wehrli, III;
Henry A. (Monroeville, PA), Smith; James D. B.
(Monroeville, PA) |
Assignee: |
Eaton Corporation (Cleveland,
OH)
|
Family
ID: |
22953791 |
Appl.
No.: |
08/251,880 |
Filed: |
June 1, 1994 |
Current U.S.
Class: |
361/58;
361/27 |
Current CPC
Class: |
H02H
3/025 (20130101); H02H 9/026 (20130101) |
Current International
Class: |
H02H
3/02 (20060101); H02H 9/02 (20060101); H02H
003/093 () |
Field of
Search: |
;361/92,58,57,27,56,23,106 ;219/546 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
PTC Application Notes, Keystone Carbon Company Bulletin T-929, pp.
34-42. .
PCT/SE91/00076, Aug. 22, 1991, Hansson et al. .
PCT/SE91/00375, Dec. 12, 1991, Hansson et al. .
PolySwitch.RTM. TR Series, Jan. 1993, 4 pages. .
Klockner-Moeller Main Catalogue (HPL 90/91 GB, FLs/Br), pp.
9/1-9/28, published May 1990 in the Federal Republic of
Germany..
|
Primary Examiner: Gaffin; Jeffrey A.
Assistant Examiner: Medley; Sally C.
Attorney, Agent or Firm: Moran; Martin J.
Claims
What is claimed:
1. A circuit controller apparatus for selectively connecting a
power source to a load, said apparatus comprising:
non-overcurrent protective electrical switching means for
selectively connecting said power source to said load and switching
a current which flows from said power source and through said
non-overcurrent protective electrical switching means; and
conductive polymer current limiting means connected in series with
said load for limiting the current which flows through said
non-overcurrent protective electrical switching means whenever said
power source is selectively connected to said load and for
substantially limiting a short circuit current which flows from
said power source, said conductive polymer current limiting means
being connected in series with said nonovercurrent protective
electrical switching means between said power source and said load,
and exhibiting a first lower electrical resistance when conducting
a first rated current, the first lower electrical resistance
rapidly increasing to a second substantially higher electrical
resistance when conducting a second current which is higher than
the first rated current, in order that said conductive polymer
current limiting means substantially limits all of the short
circuit current which flows from said power source, said conductive
polymer current limiting means including plurality of conductive
polymer positive temperature coefficient (PTC) resistors which are
each electrically connected in parallel with each other of the
conductive polymer PTC resistors, said conductive polymer current
limiting means including at least two conductive plates, and
wherein each of the plurality of conductive polymer PTC resistors
includes two terminals, each of the two conductive plates being
electrically connected to a corresponding one of the two terminals
of each of the plurality of conductive polymer PTC resistors.
2. A circuit controller apparatus for selectively connecting a
power source having a voltage to a load, said apparatus
comprising:
electrical switching means having separable contacts and a coil,
the separable contacts being connected in series between said power
source and said load, the coil being energized to close the
separable contacts in order to connect said power source to said
load and being deenergized to open the separable contacts in order
to disconnect said power source from said load;
conductive polymer current limiting means connected in series
between said power source and said electrical switching means for
limiting the current which flows through the separable contacts
whenever said power source is selectively connected to said load
and for substantially limiting a short circuit current which flows
from said power source, said conductive polymer current limiting
means having an output which has a voltage substantially less than
the voltage of said power source and through said conductive
polymer current limiting means in order that the voltage of said
power source is substantially across said conductive polymer
current limiting means; and
voltage sensing means connected to the output of said conductive
polymer current limiting means for deenergizing the coil of said
electrical switching means, in order to open the separable contacts
of said electrical switching means and disconnect said power source
from said load whenever the voltage of the output drops below a
predetermined threshold, said conductive polymer current limiting
means having a first electrical resistance when conducting a first
rated current, the first lower electrical resistance rapidly
increasing to a second substantially higher electrical resistance
when conducting a second current which is higher than the first
rated current, in order that said conductive polymer current
limiting means substantially limits all of the short circuit
current which flows from said power source, said conductive polymer
current limiting means including a plurality of conductive polymer
positive temperature coefficient (PTC) resistors which are each
electrically connected in parallel with each of the plurality of
conductive polymer PTC resistors, said conductive polymer current
limiting means including at least two conductive plates, and
wherein each of the plurality of conductive polymer PTC resistors
includes two terminals, each of the two conductive plates being
electrically connected to a corresponding one of the two terminals
of each of the plurality of conductive polymer PTC resistors.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
The inventions taught herein are related to a concurrently filed
commonly assigned application Ser. No. 08/251,873 entitled "LOW
COST CIRCUIT CONTROLLER" by James A. Bauer et al. (Attorney Docket
No. 94-ECC-051). The inventions taught herein are further related
to commonly assigned application Ser. No. 08/161,017 entitled
"COMBINATION LOAD CONTROLLER" (Attorney Docket No. 57,523) filed
Dec. 3, 1993 and application Ser. No. 08/139,679 entitled "SANDWICH
CONSTRUCTION FOR CURRENT LIMITING POSITIVE TEMPERATURE COEFFICIENT
PROTECTIVE DEVICE" (Attorney Docket No. 57,767) filed Oct. 22,
1993.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an electrical circuit controller for
protecting a power circuit and a load powered by the power circuit,
and more particularly to a contactor having an external current
limiting conductive polymer resistance for providing current
limiting protection in the power circuit.
2. Background of Information
Electromagnetic contactors are electrically operated switches used
for controlling motors and other types of electrical loads.
Contactors include a set of movable electrical contacts which are
brought into contact with a set of fixed electrical contacts to
close the contactor and connect a power line to the load. The set
of movable contacts are separated from the set of fixed contacts to
open the contactor.
Contactors also include a magnetic circuit having a fixed magnet
and a movable armature with an air gap therebetween when the
contactor is opened. An electromagnetic coil is controllable upon
command to interact with a source of voltage for
electromagnetically accelerating the armature towards the fixed
magnet, thus reducing the air gap. Disposed on the armature is the
set of movable contacts. The complementary set of fixed contacts
are fixedly disposed within the contactor case and engage the
movable contacts as the magnetic circuit is energized and the
armature is moved. The load and voltage source therefor are
interconnected with the fixed contacts and become interconnected
with each other as the movable contacts make with the fixed
contacts.
Contact erosion in contactors primarily occurs during a contact
breaking cycle. During such cycle, the separable contacts (i.e.,
the fixed contacts and the movable contacts) part and the current
flowing therethrough forms an arc. Continued arcing eventually
interferes with the ability of the separable contacts to conduct
electricity. The surface of the separable contacts may become
eroded, pitted or may have carbon build-up.
A motor-starting contactor with a thermal overload protection relay
system is called a motor starter. The purpose of an overload relay
is to sense heat produced by line current and "trip" or stop the
motor if the retained heat exceeds an acceptable level. State of
the art overload protection relay systems include current sensors
which output a voltage proportional to the current. After an
analog-to-digital conversion of the voltage, a microprocessor
squares and integrates the converted digital value to achieve a
true measure of motor heating. This approach provides an accurate
degree of motor protection. However, the overload relay cannot
effectively interrupt the entire short circuit current. Without
additional circuit protection, the short circuit current may damage
the separable contacts of the contactor.
Circuit breakers are generally used to protect electrical circuitry
from damage due to an overcurrent condition, such as an overload
fault or a relatively high level short circuit condition. An
overload fault condition is normally about 125-600 percent of the
nominal current rating of the circuit breaker. A high level short
circuit condition can be 1000 percent or more of the nominal
current rating of the circuit breaker. For example, in a bolted
three phase line-ground fault, the short circuit current may exceed
100 KA at 480 VAC.
In conventional motor starter installations, a three-phase power
source powers a motor through a circuit breaker and a separate
motor starter having an overload protection relay. The circuit
breaker generally provides both overcurrent protection and power
circuit disconnection functions. Under short circuit fault
conditions, the circuit breaker acts first to protect the power
circuit. This is because the motor starter trip characteristics are
generally designed for interrupting, after an I.sup.2 t time delay
period, persistent overload currents associated with motor
overloads. In the event of a motor starter failure, involving
welded contacts, the circuit breaker may also be used to disconnect
the power source from the motor.
Positive temperature coefficient (PTC) resistive elements are known
to be used as reusable fuses. A PTC resistive element exhibits a
relatively low resistance to a flow of electrical current when the
current is below a threshold value. Current above the threshold
value flowing through the PTC resistive element causes resistive
heating of the PTC element. A rise in internal temperature of the
PTC resistive element to above a transition temperature causes the
PTC element to transition into a state of high resistance, thereby
limiting current flow through the PTC element and the circuit
containing it.
As stated in PTC Application Notes, Keystone Carbon Company
Bulletin T-929, page 37, "[t]he dramatic rise in resistance of a
PTC at the transition temperature makes it an ideal candidate for
current limiting applications. For currents below the limiting
current (I.sub.L), the power being generated in the unit is
insufficient to heat the PTC to its transition temperature.
However, when abnormally high-fault currents flow, the resistance
of the PTC increases at such a rapid rate that any further increase
in power dissipation results in a reduction in current."
Examples of PTC resistive devices include silicon carbide,
tungsten, polycrystalline ceramic barium titanate, barium and
strontium titanate, and current limiting conductive polymers.
Current limiting conductive polymers are known in the art to be
useful for limiting electrical current. For example, Raychem
Corporation manufactures and markets a current limiting polymer
under the trademark PolySwitch.TM.. Current limiting polymers
having PTC characteristics are disclosed in U.S. Pat. Nos.
4,545,926; 4,560,498 and 4,775,778, all owned by Raychem
Corporation. Current limiting polymers typically comprise
cross-linked polyethylene, heavily doped with carbon. PTC's
typically have a low electrical resistance when conducting current
below a threshold value (i.e., when the PTC is relatively cool).
When current flowing through the PTC exceeds the threshold value,
resistive heating produces a rise in the internal temperature of
the PTC, causing a reduction in conductivity (i.e., an increase in
electrical resistance). The power dissipated in the PTC is
proportional to the resistance multiplied by the square of the
current. Therefore, for a constant or increasing current flow, an
increase in resistance leads to a further increase of resistance.
This increase in resistance is, thus, quite rapid. Typically, the
increase in resistance is virtually a step function once the
magnitude of the current (and the resulting internal temperature of
the polymer) surpasses the threshold value.
The change in resistance of the PTC upon passing the threshold
value is quite large. For example, the resistance of a current
limiting polymer upon passing the threshold value may rapidly
increase by a factor of 1,000 to greater than 4,000 times its
initial resistance. Whenever the PTC is connected in a power system
in series with a load, the increase in resistance of the PTC
increases the total load resistance and, thus, substantially
reduces the load current. However, the increase in resistance of
the PTC produces a corresponding increase in the voltage drop
across the PTC and a corresponding decrease in the voltage drop
across the load. In this manner, a larger portion of the power from
the power line is dissipated in the PTC as heat, as opposed to
being dissipated by the load. Depending on the application (i.e.,
the line voltage and the load resistance), the voltage drop across
a PTC which has transitioned to a high resistance state may be
substantial and may result in destruction of the PTC. This is
especially true when a conductive polymer is used as the PTC.
Furthermore, PTC's are known to exhibit negative temperature
coefficient (NTC) resistance characteristics if the internal
temperature of the PTC goes much beyond the PTC threshold value. If
heated to the NTC threshold value, the resistance of the PTC
decreases.
Placing a PTC in thermal communication with a heat producing
component is known in the art. Such an arrangement is disclosed in
U.S. Pat. Nos. 4,780,598 and 5,064,997, both owned by Raythem
Corporation. As disclosed in U.S. Pat. Nos. 4,780,598 and
5,064,997, the heat producing component is a voltage-dependent
resistor. The voltage-dependent resistor and the PTC are
electrically coupled in a series circuit with components to be
protected from excessive current flow. The heat producing component
radiates heat to the PTC to accelerate its transition into a state
of high resistance to protect the other circuit components.
In contrast to the use of circuit breakers which interrupt short
circuit fault currents, various proposals have been advanced for
limiting such fault currents in conventional motor starter
installations. International Application Number PCT/SE91/00076
discloses a circuit breaker comprising a trip circuit, a trip coil,
a set of contacts responsive to the trip coil, and a positive
temperature coefficient conductive polymer thermistor in series
with the set of contacts. The thermistor limits the magnitude of
the short-circuit current. However, the coil of the trip circuit
rapidly opens the contacts within approximately 5 ms after a fault
current exceeds five to ten times rated current.
It is well known that conductive polymers have a relatively limited
heat capacity characteristic. Thus, prior art proposals utilize a
separate overcurrent protective circuit, such as a circuit breaker
trip coil or a motor starter overload relay, to open the power
circuit within two power line cycles. In this manner, the
conductive polymer current limiter is protected from excessive
temperatures during short circuit fault conditions. Therefore, the
prior art proposals which use conductive polymers either require a
circuit breaker for rapidly interrupting the short circuit fault
current or, else, require a contactor having an overload relay or a
special sensor for interrupting the current flow after a limited
number of power line cycles. In either case, the circuit breaker,
the overload relay or the sensor must act relatively quickly (i.e.,
within a few power line cycles) after a short circuit fault in
order to protect the conductive polymer current limiter.
Prior art applications, of necessity, rapidly interrupted the short
circuit fault current before the temperature of the conductive
polymer rose to a destructive level. In the same manner, such prior
applications rapidly interrupted the short circuit fault current
before the resistance of the conductive polymer rose to a
sufficient resistance which divided substantially all of the power
line voltage across the conductive polymer. Furthermore, such prior
applications, of necessity, rapidly interrupted the short circuit
fault current flowing through the power circuit before the current
and load side voltage were substantially reduced. Accordingly,
contact arcing and contact erosion of the separable contacts of the
contactor persist.
There is a need, therefore, for a contactor which minimizes contact
arcing and contact erosion under short circuit conditions.
There is a more particular need for such a contactor which protects
a power circuit from damage under short circuit conditions.
There is an even more particular need for such a contactor which is
not damaged or seriously degraded under short circuit
conditions.
There is another even more particular need for a circuit controller
which protects a power circuit from damage without requiring a
circuit breaker.
SUMMARY OF THE INVENTION
These and other needs are satisfied by the invention which is
directed to a circuit controller for selectively connecting a power
source to a load. The circuit controller includes a non-overcurrent
protective electrical switching mechanism for selectively
connecting the power source to the load and a conductive polymer
current limiter connected in series with the load. The
non-overcurrent protective electrical switching mechanism switches
a current which flows from the power source and through the
non-overcurrent protective electrical switching mechanism. The
conductive polymer current limiter limits the current which flows
through the non-overcurrent protective electrical switching
mechanism whenever the power source is selectively connected to the
load and substantially limits all of the short circuit current
which flows through the non-overcurrent protective electrical
switching mechanism from the power source.
The conductive polymer current limiter, which is connected in
series with the non-overcurrent protective electrical switching
mechanism between the power source and the load, may exhibit a
first lower electrical resistance when conducting a first rated
current. This first lower electrical resistance may rapidly
increase to a second substantially higher electrical resistance
when conducting a second short circuit current which is higher than
the first rated current. A ratio of the second substantially higher
electrical resistance to the first lower electrical resistance may
exceed 4,000. The conductive polymer current limiter may include a
plurality of parallel conductive polymer positive temperature
coefficient (PTC) resistors.
Under short circuit conditions, the electrical switching mechanism
remains selectively connected to the load. The short circuit
current which is limited by the conductive polymer current limiter
continues to flow through the electrical switching mechanism. The
electrical switching mechanism may include a contactor having
separable contacts. Alternatively, the electrical switching
mechanism may further include a circuit interrupter which is
connected in series with the separable contacts of the contactor,
in order to provide a separate circuit disconnection mechanism. The
contactor may be a motor-starting contactor.
Alternatively, the circuit controller may include an electrical
switching mechanism having separable contacts and a coil, a
conductive polymer current limiter connected in series between the
power source and the electrical switching mechanism, and a voltage
sensor. The conductive polymer current limiter substantially limits
all of the short circuit current which flows from the power source.
An output of the conductive polymer current limiter has a voltage
substantially less than the voltage of the power source whenever
the short circuit current flows from the power source and through
the current limiter. The voltage sensor is connected to the output
of the conductive polymer current limiter. Whenever the output
voltage of the conductive polymer current limiter drops below a
predetermined threshold, the voltage sensor deenergizes the coil of
the electrical switching mechanism, in order to open the separable
contacts of the electrical switching mechanism and disconnect the
power source from the load.
BRIEF DESCRIPTION OF THE DRAWINGS
A full understanding of the invention can be gained from the
following description of the preferred embodiment when read in
conjunction with the accompanying drawings in which:
FIG. 1 is a block diagram of a circuit controller in accordance
with the invention;
FIG. 2 is a block diagram of a circuit controller in accordance
with an alternative embodiment of the invention;
FIG. 3 is an isometric view of a conductive polymer current limiter
having a plurality of positive temperature coefficient (PTC)
conductive polymer resistors in accordance with the invention;
FIG. 4 is a graph of PTC conductive polymer resistance with respect
to temperature in accordance with the invention; and
FIG. 5 is a block diagram of a circuit controller in accordance
with another alternative embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a block diagram of a circuit controller 2 is
illustrated. The exemplary controller 2 is configured in an
exemplary power circuit 4 having a three-phase alternating current
(AC) power source 6 and a three-phase AC load 8, it being
understood that the invention is applicable to controllers, power
sources and loads having any number of phases, and furthermore, it
being understood that the invention is applicable to direct current
(DC) controllers, power sources and loads. The controller 2
selectively connects three power line phases LINE A,LINE B,LINE C
of the power source 6 to three power inputs LOAD A,LOAD B,LOAD C,
respectively, of the load 8. The controller 2 includes three
current limiters 10A,10B, 10C and an exemplary contactor 12 having
three separable contacts 14A,14B,14C for switching the flow of
current from the power source 6 through the contactor 12. The
contactor 12 also includes an electromagnet 15 and a contactor coil
16. Whenever the coil 16 is energized, the separable contacts
14A,14B,14C are first closed and then are held closed in order to
connect the three-phase power source 6 to the three-phase load 8.
On the other hand, whenever the coil 16 is deenergized, the
separable contacts 14A,14B,14C are opened, thereby disconnecting
the power source 6 from the load 8.
Electromagnetic contactors are well known in the art; an example is
disclosed in U.S. Pat. No. 5,315,471, issued May 24, 1994 to Rick
A. Hurley et at. entitled "Coil Current Regulator with Induced Flux
Compensation in an Electromagnetic Contactor System", which is
herein incorporated by reference, it being understood that the
present invention is applicable to a wide variety of
electromagnetic switching devices, such as vacuum, air gap and
insulating gas contactors or motor starters. A complete description
of a motor-starting contactor having an overload relay is disclosed
in U.S. Pat. No. 4,893,102, issued Jan. 9, 1990, which is herein
incorporated by reference.
The current limiters 10A, 10B, 10C are connected in series with the
power inputs LOAD A,LOAD B,LOAD C, respectively, of the load 8 and
limit the three-phase currents which flow through the controller 2
whenever the power source 6 is selectively connected to the load 8.
Three power circuits A,B,C for the three phases are formed by the
series connection of the power lines LINE A,LINE B, LINE C, the
current limiters 10A,10B,10C, the separable contacts 14A,14B,14C,
and the power inputs LOAD A,LOAD B,LOAD C, respectively.
Referring now to FIG. 2, a block diagram of an alternative
embodiment of the circuit controller is illustrated. In this
alternative embodiment, a circuit controller 18 includes an
electromagnetic switching device, such as the exemplary contactor
12, the current limiters 10A,10B,10C, and a separate disconnection
mechanism, such as the exemplary circuit interrupter 20. The
circuit interrupter 20 includes three separable contacts
22A,22B,22C which are connected in series between the three power
lines LINE A,LINE B,LINE C and the current limiters 10A,10B,10C,
respectively. The principal function of the circuit interrupter 20
and the separable contacts 22A,22B,22C is to provide a mechanism
independent of the separable contacts 14A, 14B, 14C for
disconnecting the three-phase power source 6 from the load 8.
FIG. 3 illustrates an isometric view of the exemplary conductive
polymer current limiter 10 for one of the power circuits A,B,C of
FIGS. 1 and 2. The current limiter 10 includes two rectangular
conductive plates 24,26 which are electrically connected to two
terminals 28,30, respectively. The terminals 28,30 are attached to
the respective plates 24,26 on opposite sides of the current
limiter 10. The terminals 28,30 are electrically connected in
series between the power source 6 and the contactor 12. The
exemplary current limiter 10 further includes a plurality of
positive temperature coefficient (PTC) conductive polymer resistors
32. The exemplary resistor 32 is manufactured and marketed by
Raychem Corporation under order number PolySwitch.TM. TR600-150, it
being understood that the invention is applicable to a variety of
PTC resistive devices such as silicon carbide, tungsten,
polycrystalline ceramic barium titanate, or barium and strontium
titanate.
Each of the plural resistors 32 has two leads 34,36 which are
connected to the plates 24,26, respectively, and, hence, each of
the plural resistors 32 is connected in parallel with each of the
other plural resistors 32. The exemplary current limiter 10
includes 76 of the exemplary resistors 32 which are generally
configured in a seven by eleven array between the plates 24,26. The
77th position of the exemplary array being used for the position of
the terminals 28,30. Accordingly, the parallel combination of the
resistors 32 is connected in series with the contactor 12 between
the power source 6 and the load 8. Table I, below, provides various
dimensions for the exemplary current limiter 10.
TABLE I ______________________________________ DIMENSION OF ITEM
(inches) ______________________________________ HEIGHT OF PLATES
24,26 4.0 WIDTH OF PLATES 24,26 4.0 THICKNESS OF PLATES 24,26 0.125
HEIGHT OF RESISTOR 32 0.354 WIDTH OF RESISTOR 32 0.531 THICKNESS OF
RESISTOR 32 0.236 THICKNESS OF CURRENT LIMITER 10 0.5
______________________________________
As described in greater detail below, the exemplary current limiter
10 has a positive temperature coefficient (PTC) resistance which
increases with corresponding increases in temperature of the
current limiter 10. Furthermore, changes in the temperature of the
current limiter 10 are determined by resistive (I.sup.2 R) heating
of the current limiter 10. In other words, the current limiter 10
exhibits a relatively low resistance to a flow of electrical
current when the current is below a threshold value (e.g., when the
current is less than or equal to the rated current of the contactor
12). However, current above the threshold value (e.g., a short
circuit fault current) flowing through the current limiter 10
causes further resistive heating and a corresponding rise in
internal temperature of the current limiter 10. In turn, this
causes the current limiter 10 to rapidly transition to a state of
substantially higher resistance. In this manner, the conductive
polymer current limiter 10 substantially limits all of the short
circuit current which flows from the power source 6 and through the
contactor 12.
During normal operation, the resistances of the plural resistors 32
remain relatively low. The parallel connection of the plural
resistors 32 reduces the resistance of the exemplary current
limiter 10 by a factor of 76 below the resistance of each resistor
32. Furthermore, since the resistance is low, the power line
voltage drop across the resistors 32 also is relatively low. As
with most conductors, an increase in current passing through the
resistors 32 causes a related rise in the internal temperature of
the resistors 32. Whenever the internal temperature of one of the
resistors 32 is increased above a threshold temperature, the
resistance of such resistor 32 increases substantially. The
exemplary power source 6 may source, for example, a per phase short
circuit current of 4.5 KA at 220 VAC (rms). Under ambient
temperature conditions, the exemplary conductive polymer current
limiter 10 limits such short circuit current to 600 A peak and 15 A
peak under steady state short circuit conditions. Table II, below,
provides resistance values for the exemplary current limiter 10
under various conditions.
TABLE II ______________________________________ CONDITION (.OMEGA.)
______________________________________ INITIAL OR NOMINAL 0.118
PEAK RESISTANCE 475 STEADY STATE SHORT CIRCUIT 20
______________________________________
Referring now to FIGS. 3 and 4, FIG. 4 illustrates a graph of
resistor temperature (.degree.C.) versus log.sub.10 resistance
(.OMEGA.) for resistor 32. As illustrated by FIG. 4, the initial or
nominal resistance of approximately 9 .OMEGA. (i.e., 0.95 in the
graph) corresponds to a wide temperature range (i.e., from less
than 20.degree. C. through approximately 125.degree. C.). The
nominal resistance also corresponds to currents flowing through the
contactor 12 which are less than or equal to the rated current of
the contactor 12. Whenever a short circuit fault current flows
through the current limiter 10, the temperature of each of the
resistors 32 rapidly exceeds 125.degree. C. with approximately 2
ms. In turn, the resistance of each resistor 32 rapidly rises to
more than 36,000 .OMEGA. (i.e., greater than 4.56 in the graph) as
the temperature rises to greater than 160.degree. C. The ratio of
the peak resistance to the initial resistance is greater than
4,000. Under steady state short circuit conditions, the steady
state resistance of the current limiter is 20 .OMEGA..
Referring again to FIG. 1, contact arcing and contact erosion of
the separable contacts 14 are substantially reduced because the
short circuit fault current is substantially reduced by the current
limiter 10. Furthermore, under steady state short circuit
conditions, the voltage of the power source 6 is substantially
across the current limiter 10 (e.g., in the exemplary embodiment,
the 15 A peak current flowing through the steady state resistance
of 20 .OMEGA. results in a 300 V peak voltage which is about 96% of
the 311 V peak voltage of the exemplary 220 VAC (rms) power source
6). In this manner, whenever the coil 16 is deenergized and, hence,
the separable contacts 14 of the contactor 12 are opened, the
switched voltage at the power inputs LOAD A,LOAD B,LOAD C of the
load 8 is also substantially reduced.
In many applications, it is preferred to open the separable
contacts 14 under any short circuit fault condition. In this
manner, such fault in the power circuit 4 may be identified and
corrected. Referring now to FIG. 5, a block diagram illustrates
another alternative embodiment of the circuit controller which
opens the separable contacts 14 under a short circuit fault
condition. In this alternative embodiment, a circuit controller 38
includes an electromagnetic switching device, such as the exemplary
contactor 12, the current limiters 10A,10B,10C, and a relay 40
having a normally-open contact 41 and a coil 42. Two external
voltages "E" and "C" power the contactor 12 which connects the
voltage "C" to an internal ground reference.
The coil 16 of the contactor 12 is selectively powered by a field
effect transistor (FET) drive circuit 43. Alternating current from
the voltages "E" and "C" is rectified by a full-wave bridge 44 and
is limited by a resistor 46. Direct current from the bridge 44
selectively energizes the coil 16 of the contactor 12 whenever the
FET drive circuit 43 is enabled. In the exemplary embodiment, an
external transformer 48 is used to generate the voltages "E" and
"C". The primary of the transformer 48 is connected to two of the
outputs (e.g., terminal 30A and terminal 30B in the exemplary
embodiment) of the three current limiters 10A,10B,10C. The
secondary of the transformer 48 generates an exemplary 120 VAC line
voltage between the voltages "E" and "C".
The coil 42 of the exemplary relay 40 monitors the voltage across
two of the outputs (e.g., terminal 30B and terminal 30C in the
exemplary embodiment) of the three current limiters 10A,10B, 10C.
Whenever the voltage across the terminals 30B and 30C drops below a
threshold voltage (e.g., whenever current limiter 10B or current
limiter 10C transitions to a state of higher resistance as a result
of a short circuit fault current) the contact 41 of the relay 40
opens and the voltage "E", which provides power to the contactor
12, is disabled. Similarly, whenever the voltage across the
terminals 30A and 30B drops below a threshold voltage (e.g.,
whenever current limiter 10A or current limiter 10B transitions to
a state of higher resistance as a result of a short circuit fault
current), the power to the contactor 12 is also disabled.
After any one of the current limiters 10A, 10B, 10C transitions to
a state of higher resistance as a result of a short circuit fault
current, the voltage of the power source 6 is substantially across
such current limiter 10 and the voltage at the corresponding output
30 is substantially reduced. In this manner, in the exemplary
embodiment, either the transformer 48 or the relay 40 detects such
short circuit fault condition. Subsequently, the coil 16 of the
contactor 12 is deenergized, thereby opening the separable contacts
14A,14B,14C and disconnecting the power source 6 from the load
8.
Those skilled in the art will appreciate that other equivalent
methods of deenergizing the coil 16 of the contactor 12 are
possible. For example, three individual relays (not shown) having
coils and normally-open contacts may be provided for each of the
outputs 30A,30B,30C of the current limiters 10A,10B,10C,
respectively. Each of the relay coils is connected between one of
the outputs 30A, 30B,30C and a power source common (not shown). The
normally-open contacts of such relays are connected in series
between the secondary of the transformer 48 and the voltage "E". In
this manner, the coil 16 of the contactor 12 is deenergized
whenever any of the current limiters 10A,10B,10C transitions to a
state of higher resistance as a result of a short circuit fault
current and the voltage at the corresponding output 30A,30B,30C
drops below a threshold voltage.
Accordingly, the exemplary circuit controllers 2,18,38 of FIGS. 1,
2 and 5, respectively, provide an improvement in cost, reliability
and operational lifetime (i.e., 10 to 100 short circuit current
limiting operations) in comparison to conventional circuit breakers
(i.e., 3 operations) or fuses (i.e., 1 operation). Moreover, the
exemplary current limiter 10 provides a reduction in enclosure
space (i.e., the size is independent of the available short circuit
fault current), improved current limiting, an elimination of
contact welding of the separable contacts 14, better load
protection, and an elimination of plasma and excessive visual
display caused by contact arcing under short circuit
conditions.
While specific embodiments of the invention have been described in
detail, it will be appreciated by those skilled in the art that
various modifications and alternatives to those details could be
developed in light of the overall teachings of the disclosure.
Accordingly, the particular arrangements disclosed are meant to be
illustrative only and not limiting as to the scope of the invention
which is to be given the full breadth of the appended claims and
any and all equivalents thereof.
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